1. Field of the Invention
The present invention relates to a drive circuit of piezoelectric elements, a driving method thereof, a liquid ejection apparatus and an image forming apparatus, more particularly to a drive technology of a piezoelectric element which is a pressurizing device for an ejection head, the technology being used in an inkjet recording apparatus and the like.
2. Description of the Related Art
As an example of the image forming apparatus, there is known an inkjet recording apparatus which has an inkjet head (ejection head) having disposed multiple nozzles (ejection elements) therein, and forms an image on a medium (ejection receiving medium) by causing the nozzles to eject ink while relatively moving the inkjet head and the medium.
There are various ink ejection methods in an inkjet head of the inkjet recording apparatus. For example, there is known a piezoelectric method where a diaphragm constituting a part of a pressure chamber is deformed by deformation of a piezoelectric element to change the capacity of the pressure chamber, followed by introduction of ink from an ink supply path into the pressure chamber when the capacity of the pressure chamber is increased, and then the ink inside the pressure chamber is ejected as droplets from the nozzle when the capacity of the pressure chamber is reduced. Also there is known a thermal inkjet method where ink in an ink chamber (pressure chamber) is heated to generate bubbles, and then the ink is ejected with the inflation energy occurring when the bubbles grow.
A pressurizing device, such as a piezoelectric element or a heater, which pressurizes ink ejected from the nozzle is driven by means of a switching element such as a transistor. The switching element is turned on or off using a drive signal corresponding to the image data, in response to which the electric power is supplied to the pressurizing device, whereby ejection energy provided to the ink from the pressurizing device is generated. When the ejection energy acts on the ink inside the pressure chamber, the ink is ejected from the nozzle.
However, in an inkjet head such as a full line type inkjet head having multiple nozzles, the number of the switching elements, along with which the number of signal lines that transmit drive signals to the switching elements increases. Such increase of the switching elements and the signal lines contributes to expansion of the head, and has an impact on the increase in electric power consumption and the cost. Therefore, in the inkjet having multiple nozzles, various measures are exercised in order to reduce the switching elements and signal lines.
As an example of reduction of wiring, there are proposed a method for driving grouped nozzle by means of a common signal line for each group, and a method for sharing a signal line by means of a signal line configured in a form of matrix.
Japanese Patent Application Publication No. 11-208000 discloses a printing method, which includes supplying ink from an ink reservoir through an ink channel that connects the ink reservoir with ink ejection chambers formed on a first surface of a substrate. The ink channel is connected at a first end to the ink reservoir and at a second end to a separate inlet passage for refilling each of the ink ejection chambers with ink. A group of the ink ejection chambers in adjacent relationship forms one of a plurality of primitives on the first surface of the substrate in which only a maximum of one of the ink ejection chambers is energized at a time. An ejection element within one of the ink ejection chambers is energized to cause the plurality of ink drops to be ejected onto a media surface at a single pixel location in a single pass of the substrate over the media surface. The plurality of ink drops are maintained as substantially separate drops until the plurality of ink drops merge upon impact with the media. Thereby, the inkjet hardcopy apparatus achieves photographic image quality.
Japanese Patent Application Publication No. 2003-320670 discloses an inkjet recording apparatus, in which a drive circuit of heater elements is constituted by a switching element which drives each of the heater elements, a level shifter which drives the switching element, an AND gate matrix circuit having an AND gate laid over a segment line as the print data and a common line which selectively scans, a segment circuit which load the print data for each print with the number of segment lines, and a common circuit which generates selective scan signals for the number of divisions obtained by dividing the total number of nozzles by the number of segment lines. The drive circuit of heater elements is a drive circuit which has a matrix configuration of the segment line and the common line and is driven in a time-shared fashion, wherein logical multiplications of the segment line and common line are generated in the AND gate, and a calculation result thus obtained is constituted so as to be converted to an application signal and driven in a matrix fashion, whereby it is possible to realize a compact inkjet recording apparatus which has high quality of drive and in which a large number of nozzles and the drive circuit can be integrally created in a long substrate.
Japanese Patent Application Publication Nos. 58-153661 and 58-153662 disclose an inkjet recording apparatus, which comprises a plurality of nozzles that are arranged in x direction and y direction, a first electrode provided on an inner wall of each nozzle or in the vicinity of each nozzle, a second electrode provided at the back of the first electrode or around each nozzle on an ink droplet chamber side on a nozzle substrate face, and a third electrode provided on a recording paper side. The inkjet recording apparatus is constituted so as to apply a pulsing voltage corresponding to image information to the first electrode, apply a pulsing voltage, which has characteristics opposite of those of the pulsing voltage applied to the first electrode, to the printing column of the second electrode, apply none or a pulsing voltage, which has the same polarity as the pulsing voltage applied to the first electrode, to a non-printed column, and apply a certain voltage, which has a reverse polarity from that of the pulsing voltage applied to the first electrode, to the third electrode, so as to stably perform the ink ejection.
Japanese Patent Application Publication No. 4-341849 discloses an inkjet print head, which is provided with a plurality of electrodes on the surface and the back face of a piezoelectric substrate, has disposed therein a nozzle plate provided with nozzles so as to correspond to the region where the surface electrode and the back face electrode intersect, and comprises ink which is held in a gap between the piezoelectric substrate and the nozzle plate, and a drive device which selectively applies a voltage to between the electrodes, the voltage changing in a cycle equal to resonance frequency of the piezoelectric substrate. Each of the electrodes is constituted so as to share a plurality of crossing regions, and makes the ink into the form of a mist by the resonance of the piezoelectric substrate to obtain a gray-scale image.
However, in the method for printing described in Japanese Patent Application Publication No. 11-208000, and the inkjet recording apparatus described in Japanese Patent Application Publication No. 2003-320670, driving elements (transistors, combinations of transistors and AND circuits, or the like), the number of which is equal to the number of pressurizing elements such as ink ejection elements or heater elements, are required, thus it is difficult to minimize the head or drive circuit.
Moreover, in the inkjet recording apparatus described in Japanese Patent Application Publication Nos. 58-153661 and 58-153662, and in the inkjet print head described in Japanese Patent Application Publication No. 4-341849, if lines are connected simply in the form of matrix, a voltage is applied to all of the rows and columns. Further, driving elements are operated using a plurality of signals having different polarities, and power supply units having different polarity and a polarity reversal circuit are required, thus it is difficult to minimize the drive circuit. In addition, the maximum difference in potential between the positive and negative voltages with different polarities has to be less than a withstanding voltage (Vp) of the piezoelectric elements. On the other hand, by applying a deviation voltage from an offset voltage, similar control is possible with the single polarity power supply; however, it is necessary to add the offset voltage and the deformation voltage to the element, whereby a voltage of the piezoelectric voltage that can be used for deformation with respect to the allowable voltage becomes small.